There are a couple of different part numbers depending on the serial number of your tractor.
These are the part numbers from the .pdf parts manual that I have.
Part Number name S/N
10242-41353 FILTER <=18523
13251-41352 FILTER >=18524
31180-35230 CAP <=11343
66591-54420 CAP >=11344
34150-35223 CAP >=18524
10211-41340 GASKET <=18523
34150-35230 GASKET >=18524
My tractor is also missing the gasket and filter, which appears to be more of a small screened fitting to trap debris when fueling from cans, than a real filter.
I can tell you that a section cut from an innertube to use as a gasket is a PITA in use. I just don't fill the tank higher than 2-3 inches from the top, and it doesn't leak. One of these days when I have to order other parts from the Kubota dealer, and have the extra cash, I will add these so I don't get bent over on shipping.
Robert